Experimental Determination of Chemical Formula

Chemical compounds are usually described using an empirical- or molecular formula. These formulas provide information about the amount of the different atoms of elements involved. But how are these formulas established?

Experimental analysis, like decomposition of compounds, is used to estimate the relative masses of constituent elements in the compound. These relative masses are then used to calculate the number of moles of each element to determine the formula of a chemical compound.

For example, a sample of a compound is experimentally determined to contain 43.64 grams of phosphorus and 56.36 grams of oxygen. Using molar masses as conversion factors, the relative masses from the experimental data are converted to 1.41 moles for phosphorus and 3.52 moles for oxygen. These mole values, when assigned as provisional subscripts to the elements, yield a pseudo-formula of the compound.

Dividing the mole values by the smallest mole value provides the mole ratios of approximately 2.5 moles oxygen to 1 mole phosphorus, which directly relates to the actual proportion of elements in the compound.

If one of the quotients is still a decimal, then all the numbers are multiplied by the smallest counting number that gives the smallest whole-number ratio of subscripts, generating the empirical formula of P₂O₅.

The molecular formula of compounds can be determined from their empirical formula and either the molar mass or molecular weight.

For example, the chemical compound with the empirical formula of P₂O₅ is experimentally measured to have a molar mass of 283.89 g/mol. Its molecular formula is a whole-number multiple of its empirical formula, while its molar mass is a whole-number multiple of its empirical formula mass.

The ratio of molar mass and empirical formula mass yields the number of formula units. Multiplying empirical formula with the number of formula units gives the molecular formula. Hence, from the molecular formula P₄O₁₀, the compound is identified as tetraphosphorus decaoxide, or more commonly known by its empirical formula name as phosphorus pentoxide.